Water spraying system

ABSTRACT

A water spraying system, especially for the humidification of the intake air of a piston engine to reduce nitrogen oxide emissions, said system comprising at least one nozzle ( 9, 10, 11, 12, 13 ) for spraying an aqueous liquid mist into the air intake duct ( 2 ) and means for conveying the liquid to be sprayed to the nozzle. The system comprises means for accomplishing the injection of a spray of aqueous liquid mist to at least one point in the air intake duct ( 2 ) depending on the load and/or speed of rotation and/or temperature of the engine.

BACKGROUND OF THE INVENTION

The present invention relates to a water spraying system as defined inthe preamble of claim 1.

The invention concerns specifically a system for supplying an aqueousliquid mist into the intake air of a preferably turbocharged pistonengine to reduce nitrogen oxide emissions (NOx). At the high combustiontemperatures, the combustion process in the cylinders of a piston engineproduces nitrogen oxides, which are emitted together with the exhaustgas into the atmosphere. Because of the harmful climatic effects ofnitrogen oxide emissions, efforts are undertaken to minimize theirproduction.

As is known, adding water to the combustion process either in the formof water vapor or water droplets reduces the formation of nitrogenoxides. This phenomenon is based on a cooling effect. When the watersprayed into the cylinder is evaporated, it reduces the temperature ofthe air in the cylinder while at the same time reducing the pressure.The pressure drop has an adverse effect on the efficiency, although thedecrease of pressure and temperature has a favorable effect on theformation of nitrogen oxides. When the water is supplied in the form ofdroplets together with the intake air, some of it is additionally wastedduring the scavenging period and water consumption is increased. Whenair saturated with water vapor is supplied into the cylinder, thethermal capacity of the filling gas is increased and the gas has asubstantially greater effect of reducing the temperatures of thecombustion process than does dry air. The effect of reducing thecombustion temperatures increases with the water vapor concentration,yet without producing an undesirable effect on efficiency. Since anincrease in the temperature of the gas supplied into the cylinder alsoaugments the generation of nitrogen oxides as well as the consumption ofwater, it is desirable to keep the gas temperature as low as possible,yet high enough to ensure that the gas supplied into the cylindercontains an amount of water vapor sufficient for the reduction ofnitrogen oxides.

An apparatus for vaporizing a desired amount of water is disclosed inpatents U.S. Pat. No. 5,758,606 and U.S. Pat. No. 6,196,165. A drawbackwith this apparatus is that the device mounted between the turbochargerand the cylinder increases the cubic volume of the air intake ductwork,which has a considerable effect on the power output of the engine. Thepower output is dependent on the cubic volume after the turbochargerbecause during power increase or decrease the air pressure produced bythe turbocharger increases the density of the air and the amount of gassupplied into the cylinder. If the cubic volume between the turbochargerand the cylinder is increased, then it will take considerably longerbefore the amount of air produced by the turbocharger brings thepressure to the desired level and the power generated by the engineincreases. Another drawback with the apparatus is that the heated waterused for vaporization and flushed over the evaporation surfaces has theeffect of increasing the temperature of the air. The device is unable tomake use of the cooling effect produced in connection with thevaporization of the water, but the gas output from the device is at arelatively high temperature, so the amount of water vapor required forthe reduction of nitrogen oxides and therefore also the waterconsumption are increased considerably.

Specification WO98/10185 again discloses an apparatus In which the airproduced by a turbocharger and the pressure of this air are utilized inthe injection of water for humidifying the air supplied to theturbocharger. A drawback with this system is the relatively lowtemperature of the supply air, which is why the amount of water vaporevaporated into the air remains small, and thus no significant nitrogenoxide reducing effect is achieved. Another drawback is that when theamount of water is increased, the water droplets can not be evaporatedafter the air has reached a saturated state, with the result that thewater droplets drift into the turbocharger and cause wear of theturbocharger vanes through droplet erosion. From a thermodynamicalviewpoint, the drifting of droplets into the turbocharger is desirableas it reduces the work performed by the turbocharger, increasing thepressure of the pressurized air produced at the output andsimultaneously reducing its temperature. In practice, however, an aircompressor rotating at a very high speed—about 50 000-100 000 rpm—hasproved to be very sensitive to droplet erosion as referred to above.

The object of the present invention is to achieve a water sprayingsystem designed for supplying water mist into the air intake ductwork ofespecially a piston engine and allowing the drawbacks of prior-art to beavoided.

The system of the invention is mainly characterized in that the systemcomprises means for producing adjustable water mist spraying at at leastone point in the air intake duct, depending on the load and/or speed ofrotation and/or temperature of the engine.

The system of the invention is additionally characterized by what isstated in claims 2-17.

The solution of the invention has numerous significant advantages. Inthe apparatus of the invention, the above-described undesirable effectsand deficiencies are eliminated by using adjustable water spraying,which is distributed to one or more points in the air intake duct byvarying the number and/or size and/or quality of the nozzles used,depending on the load and temperature of the engine. According to theinvention, the water flux is distributed to a number of small nozzles tomake it possible to produce sufficiently small droplets and/or todistribute such droplets over a larger are in the air intake duct so asto achieve an optimal vaporization. In the system of the invention, thespraying can also be focused on optimal points in the air intakeductwork where the temperature and/or air flow is highest. In the systemof the invention, the number of nozzles spraying, the point and/ordirection of injection of the spray in the air intake ductwork can bevaried according to the amount of water needed, e.g. on the basis of theload and/or speed of rotation of the engine. In the system of theinvention, it is further possible to maintain a high nozzle pressure soas to keep the droplet size of the mist being sprayed sufficientlysmall. Furthermore, the system allows the spraying to be varied betweennozzles having different properties. The system of the inventionproduces an optimal droplet size of the liquid injected into the intakeair. By using a nozzle cleaning system as part of the water sprayingsystem, very reliable operation of the system is achieved because thepossibility of nozzles being clogged is avoided. By using pop-up nozzlesas part of the system, the risk of the nozzles being clogged is furtherreduced. On the other hand, by using pop-up nozzles, the nozzles are noimpediment to the flow in the air intake duct when the system is out ofuse.

As the apparatus is connected directly to the structures of the airintake duct and it produces a fine mist directly without using any extrachambers or other containers, it is able to make full use of the heatquantity required for the vaporization of the water, cooling the intakeair at each spray injection point to a temperature close to the wet bulbtemperature (or adiabatic saturation temperature, which in the case of awater-air mixture is practically the same thing), i.e. to thetemperature to which the air temperature can be reduced by vaporizationof water. As connecting the apparatus of the invention to a turbochargedengine does not involve any changes in the cubic volume of the airintake system, it has no adverse effect on the power output of theengine, either.

Another advantage of the invention is that the humidity of the intakeair can be increased stepwise after each heat supply point, thusadjusting the humidity of the gas fed into the cylinder and thereforethe formation of nitrogen oxides within desired limits.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention will be described in detail by the aidof an example with reference to the attached drawings, wherein

FIG. 1 presents a system according to the invention in connection with aturbocharged piston engine,

FIGS. 2 a, 2 b presents sectioned view of a spraying head in the systemof the invention,

FIG. 3 a presents a diagram of a control system of the spraying systemof the invention,

FIG. 3 b presents a graph representing the operation of the system inFIG. 3 a,

FIG. 4 presents a spraying system according to the invention and acleaning system utilized in the spraying system, and

FIG. 5 presents a diagram of a system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

As stated, the invention concerns a water spraying system, especiallyfor humidification of the intake air of a piston engine 1 to reducenitrogen oxide emissions, said system comprising at least one nozzle 9,10, 11, 12, 13 for spraying an aqueous liquid mist into the air intakeduct 2 and means 21, 15, 17, 18, 9-13 for conveying a liquid to thenozzle. The system comprises means for producing adjustable waterspraying at at least one point in the air intake duct 2, depending onthe load and/or speed of rotation and/or temperature of the engine. Inthe system, the amount of water (water flux) to be injected into the airintake duct 2 is distributed to a number of nozzles 9, 10, 11, 12, 13.The water flux is typically distributed to a plurality of small nozzlesto achieve a small droplet size. The amount of water to be injected isdistributed in the air intake duct 2 over a larger area to accomplish anoptimal vaporization, preferably at points where the temperature and/orair flow rate is high, preferably highest, or in the vicinity of suchpoints. In the system, the number of nozzles spraying 9, 10, 11, 12, 13is adapted/adaptable according to the required amount of water to besprayed. The point and/or direction of injection of the water mist sprayis adapted/adaptable according to the required amount of water to beinjected. The system is designed to maintain a high nozzle pressure tokeep the spraying liquid's droplet size small. The system preferablycomprises nozzles 9, 10, 11, 12, 13 having different properties,permitting the number of and/or nozzle type of the nozzles spraying tobe varied according to the amount of water required. Several nozzles 9,10, 11; 12, 13 in the system may be arranged on the same mounting frame6, 7. The system comprises control equipment by means of which thespraying action of at least some of the nozzles 9-13 can be adjusted.

The system comprises at least one valve element 13, 14, by means ofwhich the liquid flow passage to the nozzle 9-13 is adjusted and/oropened/closed. The system comprises a control system which maintains anat least nearly constant pressure or a predetermined pressure level inat least one supply pipe 17 leading to the nozzles regardless of theoutput of the pump 15. An embodiment of the system comprises an outputregulating pump unit which controls the pressure so that the pressure inat least one supply pipe leading to a nozzle is constant. Anotherembodiment of the system employs a control system which comprises aconstant-output pump and controls the pressure by means of a valvesystem to produce a constant pressure in at least one supply pipeleading to a nozzle. The system further comprises a system for cleaningthe nozzles and/or keeping the nozzles clean.

FIG. 1 is a diagrammatic representation of a system/apparatus accordingto the invention, installed in connection with the air intake duct 2 ofa piston engine, such as a diesel engine. The air intake duct 2 and theexhaust gas duct 3 are shown in a simplified form in the figure. Theengine presented in the figure is provided with intake air compressor 4,which feeds air under positive pressure into the air intake ductwork 2of the engine. To reduce the nitrogen oxide emissions of the engine, theair intake ductwork is provided with at least one nozzle 9-13 fitted tosupply water mist into the intake ductwork 2. In the case illustrated inthe figure, the intake ductwork is also provided with a heat exchangerelement 5, such as a charge-air intercooler, which also functions as aheater.

At least one of the nozzles in the spraying system is connected directlyto the structures of the air intake duct 2, and a fine mist is produceddirectly into the intake air in the air intake duct by means of itsnozzle head 6, 7 comprising at least one nozzle. When the solution ofthe invention is used, no extra chambers or other containers need to beprovided in the air intake ductwork. The nozzles feed water mist under ahigh pressure into the air intake duct. The apparatus comprises meansfor producing the required amount of water to the desired pressure andto achieve a droplet size as favorable as possible. The pressure in theliquid supply piping is typically over 10 bar, preferably over 30 bar,most preferably over 50 bar. The pressure may be typically between10-300 bar. The liquid, especially aqueous liquid injected into the airintake ductwork is a fine mist. 50% of the water volume (Dv50) is in theform of droplets typically having a droplet size below 200 micrometers,preferably below 100 micrometers and more preferably below 50micrometers. Under high load conditions, the droplet size may be larger.

The system comprises means for supplying an aqueous liquid to thenozzles. In the embodiment presented in FIG. 1, the system comprises aliquid source 21, from where the liquid is pumped through a pipe 17 bymeans of a pump 15. The pump is operated by a drive 16. The pump istypically a high pressure pump, e.g. a displacement pump. The liquid canbe directed via channels 18, 19 to different nozzles 9-13. As shown inFIG. 1, several nozzles 9-11; 12-13 can be connected to the samemounting frame 6, 7. The mounting frame 6, 7 presented in the figure hasa separate channel for each one of the nozzles 9, 10, 11, 12, 13. Thismakes it possible to control the pressure and/or flow of the pressuremedium supplied to each nozzle 9, 10, 11, 12, 13 individually. Accordingto another embodiment of the invention, the nozzles of the spraying headcan be arranged in groups so that one feed channel leads to severalnozzles. The nozzles in the spraying head may have different properties,which have been adapted according to the placement of each nozzle. Theform of the spraying head, the number of nozzles and their orientationmay vary depending on the application. It is also possible to supplydifferent mediums to the nozzle, such as water and gas. The figure doesnot show the nozzles in detail, but they may be replaceable depending onthe application. The nozzles are therefore of a type such that theyproduce a spray of fine mist when supplied with liquid under a highpressure. Many kinds of nozzles of this category are known, e.g. fromfire extinguishing technology employing water mist. For example,specifications WO 92/20454 and WO 94/06567 disclose nozzles that producea water mist at a high pressure. Naturally, other types of nozzles mayalso be used, e.g. specification WO 01/45799 discloses yet anothernozzle.

The flow direction of the intake air in the intake ductwork 2 isindicated by an arrow in the figure.

The direction of injection of the liquid mist sprayed through thenozzles is typically so selected that a maximal relative difference ofvelocity and temperature is achieved.

The amount of water supplied through the nozzles typically increaseswith increasing engine load. Thus, when the engine load is low, it ispossible to supply water only to some of the nozzles and increase thenumber of nozzles spraying when the load increases. Similarly, thespraying head can be provided with nozzles having different properties,such as flow rate, droplet size produced by the nozzles, etc. It is thuspossible to form different combinations which can be adapted to a widerange of different applications, different engine types, differentplacements and conditions.

Typically, the amount of water supplied through the nozzles increases asthe engine load increases. This can be implemented e.g. by using acontrol system whereby the speed of rotation of the pump 16 is increasedby the drive device driving the pump. This increases the pressure in thesupply piping 17 and, based on data provided by a pressure transmitter,liquid flow passages 18, 19 are opened for more nozzles 9-13 and/or anozzle having a greater spraying capacity is engaged by opening a liquidflow passage for it. Similarly, when the load decreases, the liquid flowpassages are closed for some of the nozzles and/or a nozzle with a lowerspraying capacity is engaged. Correspondingly, an arrangement can beused such that, when the load is low, liquid is injected from nozzlesproducing a smaller droplet size, and when the load increases, thedroplet size is increased, e.g. by opening a liquid flow passage tonozzles producing larger droplets. It is also possible to use anarrangement such that, in low-load conditions involving a lowertemperature, the pressure in the supply piping is higher, smallerdroplets being thus produced by the nozzle when the load is small.

The control can also be implemented by using constant power control tocontrol the pump unit. The power of the pump unit equals the pressure inthe piping multiplied by the quantity of liquid flowing per unit of time(P[kW]=p[bar]*Q[l/min]=constant). This control mode aims at keeping thepower of the pump unit constant. When the rate of flow increases, thepressure is reduced and vice versa, so that the power remainssubstantially constant or nearly constant.

The embodiments described above are only examples. Accordingly, in someembodiments the amount of water supplied is reduced according to engineload.

The apparatus of the invention is able to make full use of the quantityof heat required for the vaporization of the water, cooling the intakeair at each spray injection point to a temperature close to the wet bulbtemperature (or adiabatic saturation temperature, which in the case of awater-air mixture is practically the same thing), i.e. to thetemperature to which it is possible to reduce the air temperature byvaporization of water.

In the apparatus of the invention, the humidity of the intake air ispreferably increased stepwise after each heat supply point. In thedirection of the intake air flow, water mist is injected before the lastheat supply point, which can advantageously be used as a waterevaporation surface. By this arrangement, the humidity of the gas fedinto the cylinder and therefore the formation of nitrogen oxides isregulated within the desired limits.

The apparatus comprises a system required for the control of the amountof water to be injected, by means of which the amount of water to beevaporated into the intake air and the cooling of the intake air can becontrolled. The apparatus comprises valve elements 13, 14 arranged inconnection with the liquid flow passages leading to the nozzles, e.g. inconnection with the pipes 18, 19. The valves 13, 14 are typicallycontrolled by a control system 20, allowing the liquid flow passages 18,19 to be opened and closed as necessary.

FIGS. 2 a and 2 b present an embodiment of a spraying head that can beutilized in the system. The spraying head 201 for humidifying the intakeair of a piston engine comprises at least one nozzle 203 and one channel211 for feeding a liquid humidifying the intake air into the air intakeduct 205 or into a space leading to the air intake duct. The sprayinghead is movable between at least two positions, a first position, inwhich first position the spraying head is retracted, and a secondposition, in which second position the spraying head 201 is protruding.In the non-active state, in the first position (FIG. 2 a), the sprayinghead is in a retracted position, whereas in the active state, in thesecond position (FIG. 2 b), at least one nozzle 203 of the spraying headextends to a position inside the air intake duct relative to the levelof the interior surface of the air intake duct 205 and/or the edges ofthe spraying head holder 202.

The holder 202 is provided with at least one guide element 213, and thespraying head, preferably its shank part 207, with at least one matingsurface 214 matching the guide element for keeping the spraying head inthe desired orientation. The guide element 213 is e.g. a groove to whicha ridge in the shank element, aligned in the direction of motion, isfitted. The guide element may also consist of e.g. rolling elements,such as balls or rollers, with a counter element 214 movably fittedbetween these.

The spraying head 201, preferably its shank part 207, and the holder 202are arranged to function as a cylinder-piston combination in which thespraying head, preferably its shank part 207, is provided with a pistonpart 206 and the holder 202 comprises a cylinder chamber 208, the pistonpart being movably fitted in it. In the embodiment in FIG. 2 a, pressuremedium is supplied from the inlet 209 through a pipe element (not shown)into the cylinder chamber space below the piston 206 from the distantend 216 of the holder 202 relative to the air intake duct 205. By theaction of the pressure medium, the piston moves upwards in the figure,thereby causing the spraying head 201 mounted on the shank part 207 tomove to the second position shown in FIG. 2 b. The pressure medium isthereby admitted from the chamber 208 via the channel 211 provided inthe shank part 207 to the nozzle 203, from where it is injected into theair intake duct. The channel 211 is typically provided with a throttleelement 217. The piston part 206 is provided with a sealing element 212or equivalent to provide a sliding fit between it and the interiorsurface of the cylinder chamber 208.

Arranged in connection with the spraying head are means for moving thespraying head 201 from the protruding position into the retractedposition. Typically, the system comprises a spring element 210 arrangedbetween the spraying head 201 and the holder 202 to move the sprayinghead from the protruding position to the retracted position. The springelement is preferably placed between the piston element 206 and the endpiece of the holder part 202 adjoining the air intake duct 205. Thespring is a helical spring, which is compressed when the nozzle headmoves into the protruding position. When the pressure of the pressuremedium acting on the piston falls below the desired value, the sprayinghead is moved to the retracted position by the action of the springforce and/or a possible pressure acting in the air intake duct.

The spraying head is secured to the wall 205 of the air intake ducttypically by a rigid joint, using e.g. fastening means 204, such asscrews or bolts, at its flange 215. The wall 205 of the air intake ductis provided with an opening for the spraying head. In the embodiment inFIG. 2 a, the spraying head, at least its nozzles 203, as seen in thedirection of motion of the spraying head, remain outside the imaginarysurface formed by the inner edges of the opening in the wall of the airintake duct, i.e. the edges on the interior side of the air intake duct.If the spraying head is of a substantially cylindrical form, then thenozzles typically open to the cylindrical surface. Similarly, if thespraying head has a conical form, then the nozzles open to the conicalsurface. Typically, the nozzles open to the lateral surface of thespraying head.

FIG. 3 a presents a diagram schematically representing the controlarrangement used in the water spraying system of the invention. Thesystem comprises at least two nozzles 301 a, 301 b, 301 c, 301 d, whichare disposed in the engine's air intake duct (not shown) or in acorresponding space leading to the combustion chamber of the engine forhumidification of the intake air. In the case presented in the figure,four nozzles are shown, with a channel 302 a, 302 b, 302 c, 302 dleading to each nozzle from a supply pipe 304 supplying a pressuremedium, preferably an aqueous liquid. The pressure medium is fed intothe supply piping by a pump 306, driven by a drive device 307. The pumppumps the pressure medium from a pressure medium source 310, such astank. Reference numbers 308 and 309 indicate a pipe and a relief valvethrough which the liquid can flow in case the pump pressure and thepressure in pipe 308 exceed a certain presettable limit value. Referencenumbers 313 and 315 indicate valves, and reference number 314 indicatesa filter. The filter prevents particles that might clog the valves 301a, 301 b, 301 c, 301 d of the spraying head from entering the sprayingsystem. When the liquid surface in the container 310 falls below acertain level, a level switch 311 will open valve 313. Switch 324 willclose the valve when the water level in the container 310 has risen to agiven height.

The pump 307 is preferably a constant-output pump which always pumps thesame amount Q of pressure medium per unit of time into the supply pipe304 when running. The pump drive is preferably a motor, such as anelectrically operated direct-current motor, which drives the pump at aconstant speed. The channels 302 a, 302 b, 302 c, 302 d leading to thenozzles are provided with valve elements A1, B1, C1, D1, which can beopened and closed as instructed by the control system. The controlsystem typically controls the valves A1, B1, C1, D1 according to therequired amount of liquid to be sprayed, preferably according to theengine load, so the amount of liquid supplied into the intake airtypically increases with the engine load. The system comprises a returnpipe 305, through which the liquid quantity not fed into the intake airreturns to the tank 310. Disposed between the supply pipe 304 and thereturn pipe 305 are valve elements A2, B2, C2, D2, which can be openedand closed as instructed by the control system. For each closed feedchannel 302 a, 302 b, 302 c, 302 d of the nozzles 301 a, 301 b, 301 c,301 d, a corresponding channel 303 a, 303 b, 303 c, 303 d opening intothe return pipe 305 is provided. If all the nozzle feed channel valvesA1, B1, C1, D1 are open, then the valves A2, B2, C2, D2 in the flowpassages leading to the return pipe 305 are closed, and vice versa. Thesum of the k-values of the return channels substantially corresponds tothe sum of the k-values of the closed nozzles and those of their feedchannels. In the embodiment represented by FIG. 3, each channel 303 a,303 b, 303 c, 303 d leading into the return pipe 305 is provided with athrottle element, which is adjusted to match the k-value of the nozzlein closed state. Thus, the sum of the k-values in the system remainssubstantially constant. In the case of FIG. 3, valve element A1 in thefeed channel 302 a leading from the supply pipe to valve 301 a is open,thus allowing the liquid to flow to the nozzle. The valves B1, C1, D1 inthe feed channels leading to the other valves are closed, thuspreventing liquid flow to valves 301 b, 301 c, 301 c. Correspondingly,valve A2 in the channel 303 a leading to the return pipe 305 is closed,preventing liquid flow through channel 303 a into the return pipe.Valves B2, C2, D2 in the other channels 303 b, 303 c, 303 d arrangedbetween the supply pipe and the return pipe are open, permitting theliquid to flow through them into the return pipe 305. The channels areprovided with a throttling 317 b, 317 c, 317 d or equivalent, whichcorresponds to the k-values of the closed nozzles. By providing nozzleshaving different characteristics and different flow rate capacities, avery large control range can be covered accurately. In the case of FIG.3, by using a pump with an output capacity of 15 l/min, where nozzle 301a has an output of 1 l/min, nozzle 301 b an output of 2 l/min, nozzle301 c an output of 4 l/min and nozzle 301 d an output of 8 l/min, theentire range of 1-15 l/min can be covered by opening and closing thevalves. FIG. 3 b visualizes the quantity of water Q supplied into theair intake duct as a function of engine load (Load). The pressure istypically constant in the system. When the engine load increases, theamount of liquid flowing into the intake air through the nozzles isincreased by increasing the number of nozzles and/or by selecting anozzle that permits a larger liquid quantity to flow through it in aunit of time. When the engine load decreases, the amount of liquidflowing through the nozzles supplying liquid into the intake air isreduced by reducing the number of nozzles and/or by selecting a nozzlethat permits a smaller amount of liquid to flow through it in a unit oftime. In connection with the above-described operation, the amount ofwater supplied into the return pipe by the “by-pass” route iscorrespondingly adjusted in inverse proportion to the amount of waterfed through the nozzles. In a corresponding manner, the throttling isadjusted so that at least when liquid is being injected into the intakeair in the system, the sum of the k-values (Σk) remains substantiallyconstant regardless of whether the liquid is passed through the nozzlesor through the return pipe or whether a portion of the liquid quantityis passed through the nozzles and another portion, substantially therest of it through the return pipe. The flow rate for a nozzle is givenby the formula Q=k✓p, where Q is the flow rate, p is the pressureforcing the medium through the nozzle and k is the nozzle resistance.The value of the factor k depends on the area of the nozzle aperture,among other things. In the case of circular aperture, the value of thefactor k depends on the aperture diameter d according to the equationk=0.78*d² when the aperture is a so-called short aperture. Theresistance of the return pipe is adapted to correspond to the resistanceof the closed nozzles. The liquid flowing back in the return pipe 305can also be circulated via a heat exchanger, which makes it possible toutilize the heat of the liquid and/or to supply more heat to it via theheat exchanger. The return pipe 305 may also be provided with a filterelement to filter out impurities from the circulated liquid. Naturallythe water coming from the liquid source can also be passed through afilter element to remove at least some of the impurities.

The system may additionally comprise means for producing pressurized airto further reduce the droplet size. The system may further compriseequipment designed for producing pressurized air, by means of which thenozzles are cleaned after use, preventing them from becoming clogged.FIG. 4 presents a cleaning system in a water spraying system, especiallyin a water spraying system designed for the humidification of intakeair, which comprises at least one spraying nozzle 401 a, 401 b, 401 c,401 d for injecting liquid into the intake air. The apparatus comprisesmeans 420, 421, 425 a, 425 b, 425 c, 425 d for supplying a secondpressure medium to the nozzle 401 a, 401 b, 401 c, 401 d after thesupply of a first pressure medium, such as a liquid and/or gas intendedfor the humidification of the intake air to the nozzle has beeninterrupted, to prevent clogging of the nozzle. The apparatus comprisesa pressure medium source, such as a pump 420 pumping pressurized air,and means for conveying the pressure medium from the pressure mediumsource to the nozzle 401 a, 401 b, 401 c, 401 d. For conveying thesecond pressure medium, a pipeline 421, 425 a, 425 b, 425 c, 425 dconnected in the nozzle feed channel 402 a, 402 b, 402 c, 402 d betweenthe valve element A1, B1, C1, D1 and the nozzle 401 a, 401 b, 401 c, 401d is used. Each pipeline 425 a, 425 b, 425 c, 425 d used for supplyingthe second pressure medium is provided with a check valve 423 to preventthe first pressure medium from flowing through the supply channel to theother nozzles and/or to the pressure medium source. The second pressuremedium is a liquid and/or gas.

FIG. 5 presents yet another embodiment of the water spraying system ofthe invention. It comprises nozzles 1 a, 1 b, 1 c, 1 d arranged in feedchannels 2 a, 2 b, 2 c, 2 d, each channel having a different number ofnozzles placed at different positions in the air intake duct K. In thisembodiment, too, the valve elements A1-A2, B1-B2, C1-C2, D1-D2controlling the liquid flow going into the nozzle feed channels 2 a, 2b, 2 c, 2 d and the return channel 3 a, 3 b, 3 c, 3 d are controlled inpairs. These valve element pairs are most appropriately controlled bymeans of solenoid valves A1′, B1′, C1′, D1′. The return channels areprovided with variable throttles 17 a, 17 b, 17 c, 17 d, by means ofwhich the flow can be adjusted as desired. Correspondingly, the pressurecan also be varied by opening and closing the throttle elements in thereturn channel. In this embodiment, the valve elements and throttles arearranged as control blocks, indicated in the figure by the number 39 anda broken line. This embodiment likewise comprises a nozzle cleaningsystem, in which a pressure medium, such as pressurized air, is suppliedfrom a pressure medium source via a pipeline 21 by means of a pump. Thepressure medium supply line 21 of the cleaning system is provided with avariable throttle element for the control of the flow. The controlsystem further comprises a temperature regulating system, whereby thetemperature of the liquid to be injected can be adjusted. The systemcomprises a heat exchanger element 33 arranged in the return line 5, towhich heat can be supplied via a line and valve 38. When a small amountof liquid is to be injected, most of the liquid quantity supplied by thepump returns back via the return line. The pressure is at least partlyconverted to heat as it passes through the throttle elements 17 a-17 d,the liquid entering the return line being thus heated. From the returnline, at least some of the liquid can be conveyed directly to the pump 6or into the tank 10. In this case, the heat exchanger element 33 may besuperfluous because the system itself generates sufficient heat in theliquid. Similarly, the heat exchanger 33 may also recover heat andtransfer it to another part. The return line 5 is also preferablyprovided with a filter element 34 for removing impurities from theliquid.

In the system of the invention, some other pressure medium, such as gas,preferably air, can also be supplied into the liquid. In this case, asolution of the same type as e.g. in Finnish patent application FI20010514, which is as yet unpublished on the date of application of thepresent application.

It is obvious to the person skilled in the art that the invention is notlimited to the embodiments described above, but that it may be variedwithin the scope of the claims presented below.

1. Water spraying system, especially for the humidification of theintake air of a piston engine to reduce nitrogen oxide emissions, saidsystem comprising at least one nozzle (9,10, 11,12, 13) for spraying anaque-ous liquid mist into the air intake duct (2) and means forconveying the liquid to be sprayed to the nozzle, characterized in thatthe system comprises means for accomplishing the injection of a spray ofaqueous liquid mist to at least one point in the air intake duct (2)de-pending on the load and/or speed of rotation and/or temperature ofthe engine.
 2. Water spraying system according to claim 1, characterizedin that the amount of aqueous liquid to be sprayed into the air intakeduct (2) is distributed in the system to several nozzles (9,10, 11,12,13).
 3. Water spraying system according to claim 1, characterized inthat the amount of aqueous liquid to be sprayed is distributed in theair intake duct (2) over a larger area to achieve an optimalvaporization, preferably to points with a high temperature and/or airflow or to their vicinity.
 4. Water spraying system according to claim1, characterized in that the number of nozzles (9,10, 11,12, 13) in thesystem is adapted according to the required amount of liquid to besprayed.
 5. System according to claim 1, characterized in that the pointof injection and/or direction of injection of the spray of liquid mistis adapted according to the required amount of aqueous liquid to besprayed.
 6. System according to claim 1, characterized in that thesystem comprises nozzles (9,10,11,12, 13) having different properties,the number and/or type of nozzles spraying being varied according to theamount of liquid required.
 7. System according to claim 1, characterizedin that the several nozzles (9-13) in the system are arranged on thesame mounting frame (6,7).
 8. System according to claim 1, characterizedin that the system comprises a regulating apparatus, by means of whichthe injection action of at least some of the nozzles (9-13) can becon-trolled.
 9. System according to claim 1, characterized in that thesystem comprises at least one valve element (13, 14), by means of whichthe liquid flow passage leading to one of the nozzles (9-13) is adjustedand/or opened/closed.
 10. System according to claim 1, characterized inthat the system comprises a regulating system, by means of which thepressure in at least one supply pipe (17) leading to the nozzles is keptat least nearly constant or at a predetermined level independently ofthe output of the pump.
 11. System according to claim 1, characterizedin that the system comprises an output regulating pump unit, by means ofwhich the pressure is regulated by pressure control so that the pressurein at least one supply pipe (17) leading to a nozzle is constant. 12.System according to claim 1, characterized in that the system comprisesa control system comprising a constant-output pump and controlling thepressure by means of a valve system to maintain a constant pressure inat least one supply pipe leading to a nozzle.
 13. System according toclaim 1, characterized in that the system further comprises a system forcleaning the nozzles and/or keeping the nozzles clean.
 14. Systemaccording to claim 1, characterized in that the pressure in the liquidsupply piping is 10-300 bar.
 15. System according to claim 1,characterized in that the droplet size of the water mist is typicallybelow 200 micrometers.
 16. System according to claim 1, characterized inthat a second pressure medium, typically a gas, preferably air, issup-plied to at least one nozzle.
 17. Apparatus according to claim 1,characterized in that the apparatus comprises means for controlling thetemperature of the liquid to be injected.